A crack where an extension meets the original house is very common, and it is usually caused by the two parts sitting on different foundations that respond differently to the ground. It may be harmless differential settlement that stopped years ago, or it may be ongoing subsidence or heave affecting the extension. A chartered structural engineer needs to establish which before any repair, and underpinning an extension is only one of several possible outcomes.
Why do extensions crack away from the house?
An extension is effectively a second building pushed up against the first. The original house might stand on shallow Victorian brick footings or inter-war strip foundations; the extension might be on deeper modern concrete, or, in older or poorly built cases, on something shallower than it should be. The two foundations carry different loads, sit at different depths and may bear on different ground. Even when both are sound, they often move very slightly differently, and the junction is where that shows.
Things get more serious when something is actively affecting one side. Common culprits include:
- Drains under or beside the extensionExtensions are frequently built over or next to existing drain runs. A cracked or displaced joint can soften clay or wash out sandy soil beneath the new footing. See drainage-related subsidence.
- Trees and shrubs on clayA rear extension brings the house closer to garden trees and hedges, so its foundation may sit within their zone of influence on shrinkable clay.
- Heave after tree removalClearing large trees or shrubs to make room for an extension can let clay soils swell back, lifting a lightly loaded floor or foundation. Our heave damage repair page explains this.
- Inadequate original foundationsSome extensions, particularly older ones built without full Building Control oversight, have footings that are too shallow for the ground.
- Poor or missing tie-inWhere the extension wall was not properly bonded or tied to the house, a straight vertical crack can open without the foundations doing anything unusual.
Which cracks around an extension matter?
A fine, straight, vertical crack at the junction that has not changed in years is often just the two structures acknowledging each other. Things that call for a closer look include a crack that is tapering (wider at top or bottom), one that keeps reopening after filling, diagonal cracking in the extension walls or around its doors and windows, a floor that has developed a slope or a gap at the skirting, a flat roof pulling away from the house wall, and new sticking of patio or bi-fold doors. RICS flags sudden cracks wider than 3 mm as the main concern.
Please don’t fill and redecorate a moving extension crack to hide it, and never conceal past movement from an insurer, buyer or lender. Photograph the crack with a ruler and the date, note any changes, and contact your buildings insurer if you suspect subsidence. On a claim, the insurer decides on investigation and repair.
How is extension movement investigated?
- Inspection. The engineer reviews the crack pattern, the construction of both parts, the age of the extension and any Building Control completion or drawings you have. A structural engineer survey sets out the likely cause and what further information is needed.
- Trial pits. Digging beside the extension and the main house, to the engineer’s instructions, shows how deep each foundation actually is and what it sits on.
- Drain survey. A CCTV camera survey checks the drain runs near or under the extension for cracks, displaced joints and root ingress.
- Soil testing and roots. Samples can show whether clay is shrinkable and whether roots are present at foundation depth.
- Monitoring if needed. Where it is unclear whether movement is continuing, crack or level monitoring over months shows the trend.
How is a moving extension repaired?
Once the cause is known, the engineer chooses the response. If a drain was to blame, it is repaired or relined and the ground allowed to recover. If vegetation is the cause and can be managed (always on arboricultural advice and with council consent where a tree is protected), monitoring may confirm that the movement has stopped. In those cases the junction and any other cracks are repaired, often by crack stitching with helical bars and a properly detailed joint.
Underpinning the extension comes in when its foundation is inadequate for the ground, or the cause cannot be removed. That might be mass concrete, mini-piles (often used where trees or deep soft ground make shallow bearing unreliable, or where access through the house is tight), or beam and base. The engineer also decides how the underpinned extension should relate to the main house, because stiffening one side can concentrate movement at the junction. In a small number of cases, demolition and rebuilding of a badly built extension on new foundations is more sensible than underpinning it.
Does underpinning an extension need permission?
Underpinning normally requires Building Regulations approval, so Building Control will be involved and will inspect excavations before concrete is placed. If the extension sits close to a neighbour’s property, especially in semis and terraces with rear extensions side by side, the Party Wall etc. Act 1996 usually needs to be considered and a party wall surveyor handles the notices. If you are selling, buyers’ solicitors commonly ask for evidence that the extension had Building Control completion in the first place, which is a separate question from the repair.
Where do we repair extension subsidence?
Extensions are everywhere on Merseyside, from rear kitchen extensions on Victorian terraces in Walton to the side and rear extensions on inter-war and post-war semis in Childwall and Maghull. We carry out engineer-designed extension repairs throughout Liverpool, Sefton, Knowsley, St Helens, Wirral and Widnes as part of our local underpinning and subsidence repair service.
What affects the cost?
The biggest variable is whether the answer is crack repair after cause removal or underpinning. Beyond that: investigation and monitoring, the extension’s size and wall length, the depth to firm ground, the method, internal disruption (kitchens are often in extensions), drain repairs, Building Control and party wall fees, and making good. Our cost guide explains these in more detail.
Crack where your extension joins the house?
Call 07777 659786 or leave your details. Let us know when the extension was built, where the cracks are and whether they are changing, and whether trees or drains are nearby. We will call back, arrange a visit and recommend an engineer’s inspection if the cause is not clear. You can also read about our wider foundation underpinning in Liverpool before you call.
Frequently asked questions
Is it normal for an extension to crack where it joins the house?
A fine, stable crack at the junction is common, because the extension and house sit on different foundations. It becomes a concern if the crack is tapering, growing, reopening after repair, or accompanied by sloping floors and sticking doors. A structural engineer can tell you which you are dealing with.
Why is my extension sinking but not the house?
Often because its foundation is shallower or sits on different ground, or because a drain beneath it is leaking, or nearby trees are drying clay soil at its foundation depth. Investigation with trial pits and a drain survey usually shows which. The repair depends on that cause.
Can an extension be underpinned without touching the main house?
Often the extension alone is underpinned, but the engineer has to think about the junction, because a newly stiffened extension can move differently from the house beside it. Sometimes the scheme extends a short way under the main wall or uses a designed movement joint.
Does home insurance cover a subsiding extension?
Buildings insurance usually covers subsidence to the home, and extensions are normally part of the building, but policies vary and a subsidence excess applies. Contact your insurer early. The insurer and its loss adjuster decide whether the claim is accepted and how repairs are carried out.
Who is responsible if the builder’s foundations were too shallow?
That depends on when the extension was built, any warranty, Building Control records and your contract with the builder. It is a matter for legal or insurance advice, not something we can decide. An engineer’s report on the foundation depth is useful evidence either way.